A vacuum-assisted countergravity casting process using a gas permeable, self-supporting mold sealingly received in a vacuum housing is described in such patents as the Chandley et al U.S. Pat. Nos. 4,340,108 issued July 20, 1982 and 4,606,396 issued Aug. 19, 1986. That countergravity casting process involves providing a mold having a porous, gas permeable upper mold member (cope) and a lower mold member (drag) sealingly engaged together at a horizontal parting plane, sealing the mouth of a vacuum housing to a surface of the mold such that a vacuum chamber formed in the housing confronts the gas permeable upper mold member, immersing the bottom side of the lower mold member in an underlying molten metal pool and evacuating the vacuum chamber to draw molten metal through one or more ingate passages in the lower mold member into one or more mold cavities formed between the upper and lower mold members.
The mold and the vacuum housing typically are sealed together to form a casting assembly using a gasket seal compressed between the bottom lip of the vacuum housing and an upwardly facing sealing surface or flange formed on the mold, either on the lower or upper mold member. Various mechanical clamping mechanisms have been provided for clamping the vacuum housing and the mold together to compress the seal therebetween; e.g., as shown in U.S. Pat. Nos. 4,340,108; 4,616,691 and 4,658,880.
The need for such mold-to-vacuum housing sealing systems complicates the casting assembly as well as the casting mold. In this latter regard, the mold must include the sealing surface/flange needed to cooperate with the gasket seal and oftentimes attachment features, such as threaded lugs, needed to cooperate with the mechanical clamping mechanism. Moreover, the need for such mechanical sealing systems limits to some extent the variety of mold designs which can be used with the system.
In the countergravity casting process described in the aforementioned patents, the lower and upper mold members typically are engaged at a horizontal parting plane therebetween in such a manner as to substantially prevent or minimize leakage of molten metal from the mold cavity at the parting plane during casting since such leakage can result in the production of unacceptable castings and damage to the vacuum housing and associated vacuum components of the casting assembly. To this end, the lower and upper mold members are often adhered (e.g., glued) together at the horizontal mold parting plane by a gluing process which is both expensive and time consuming.
Moreover, in practicing the aforementioned vacuum countergravity process, the mold is subjected to flexural and other stresses when the vacuum chamber confronting the upper mold member is evacuated and the molten metal is drawn upwardly into the mold cavity. The thickness and thus the strength of the walls of the casting mold must be sufficient to withstand these and other stresses imposed on the mold during casting to prevent cracking or total fracture of the mold and resultant molten metal leakage from the mold cavity into the vacuum chamber. A reduction in both the thickness of the mold walls and the outside structural features needed for sealing to the mouth of the vacuum chamber would reduce the amount of expensive resin-bonded sand employed in the mold and thus improve the economies of the casting process. Moreover, without such excess mold material and structural features, more of the volume of the vacuum chamber would be available to accommodate more molds and hence increase the number of castings possible per casting cycle for a given size vacuum chamber.
Recent improvements in the vacuum-assisted countergravity casting process, represented by copending applications Ser. No. 191,544 and now U.S. Pat No. 4,874,029 and Ser. No. 346,627 of George D. Chandley and of common assignee herewith, have achieved substantial increases in the productivity and economies of the process by overcoming the drawbacks discussed above; i.e., by eliminating the need for large quantities of costly mold-making particulate (e.g., resin-containing sand), the need for mechanical mold-to-vacuum housing sealing/clamping systems and the need to glue the mold members together to minimize leakage of molten metal at the parting plane therebetween. A reduction in both the thickness of the mold walls and the complexity of the exterior structural features needed on the mold has resulted from these improvements and allowed more molds/mold cavities to be housed within a given size vacuum chamber than previously possible.
In these copending applications, one or more gas permeable molds (e.g., resin-bonded sand molds) or one or more destructible patterns (e.g., polystyrene patterns) are surrounded in a mass of particulate mold material (preferably binderless foundry sand) held within an open bottom container by establishment of a suitable negative pressure differential between the interior and exterior of the container. The particulate mass and molds/patterns are held in the container such that lower molten metal inlets of the molds/patterns are exposed at the open bottom end of the container for immersion in an underlying molten metal pool while the interior of the container is evacuated to effect countergravity casting of the molten metal upwardly to replace the patterns in the particulate mass or into the cavities of the molds in the mass. After the molten metal has solidified and the metal-filled container is moved to an unload station, the vacuum in the interior of the container is released to permit ready discharge of the particulate mass, castings and molds, if used, through the open bottom end of the container.
One technique used for assembling the patterns/molds and the surrounding particulate mass in the container involves inverting the container such that its open end faces upwardly, positioning the patterns/molds in the container and then gravity filling the container with binderless foundry sand through the upwardly facing open end to surround the patterns/molds in a foundry sand mass. Thereafter, the interior of the container is evacuated to establish the required negative pressure differential to hold the foundry sand in the container about the patterns/molds upon inversion of the container to orient its open end downwardly for countergravity casting.
In another assembly technique used, an open ended can is initially placed about the patterns/molds and the particulate mold material (e.g., foundry sand) is introduced into the container through the open top end thereof and falls by gravity about the patterns/molds to surround them in the particulate mass. The particulate mass is leveled with the open top end of the container and a separate vacuum box is sealingly attached on the top of the particulate-filled container to provide a casting assembly for immersion in the underlying molten metal to effect countergravity casting.
In view of the continuing desire for improvements in the productivity and economies of the vacuum-assisted countergravity casting process, a reduction in the number of operations and equipment requirements for assembling the casting assembly would be welcomed. In particular, it would be desirable to substantially reduce the number of processing steps and the overall time required to carry out assembly of the patterns/molds and surrounding particulate mass in the open bottom container. It would also be desirable to eliminate the need for complex multi-part containers requiring sealing between components thereof as well as associated handling and clamping equipment required for multi-part containers.
It is an object of the present invention to provide an improved method of assembling one or more casting molds or destructible patterns and a surrounding particulate mass in an open bottom container to form a casting assembly for practicing the aforementioned vacuum-assisted countergravity casting process.
It is another object of the present invention to reduce the number of processing steps and the time involved in assembling casting molds/patterns and a surrounding particulate mass in the open bottom container for practicing the aforementioned vacuum-assisted countergravity casting process.
It is still another object of the invention to reduce the equipment requirements involved in assembling molds/patterns and a surrounding particulate mass in an open bottom container for practicing the aforementioned vacuum-assisted countergravity casting process.